Kaolin clay pigments used by the paper and paint industry are available in both uncalcined and calcined grades, both of which bear a negative charge when dispersed in water. When preparing aqueous coating or paper filling compositions containing such pigments, it is frequently desirable to provide the clay in the form of a concentrated suspension (slurry) which is sufficiently fluid at both high and low rates of shear to be handled by conventional mixers and pumps.
In the manufacture of paper and paper board, it is well-known to incorporate quantities of inorganic fillers into the fibrous web in order to improve product quality. Titanium dioxide is widely used to improve brightness and opacity, but it is an expensive pigment. In recent years, considerable efforts have been made to develop satisfactory replacements for titanium dioxide. Substantially anhydrous kaolin clays prepared by partially or fully calcining a fine particle size fraction of crude kaolin clay is now a replacement pigment of choice. Calcined kaolin clay opacifying pigments, such as the products supplied under the registered trademarks ANSILEX and ANSILEX 93 by Engelhard Corporation are exemplary. These products are substantially anhydrous white pigments and are widely used as fillers in paper sheets and paper board, as a coating pigment for paper, and as a pigment in paints and other filled systems. They consist of aggregates of clay particles, and exhibit exceptionally high light-scattering and opacifying characteristics when incorporated as a filler into paper. The particle size of these pigments is typically at least 65 percent by weight finer than 2 micrometers equivalent spherical diameter (ESD), and at least 50 percent by weight finer than 1 micrometer. The pigments exhibit low Valley abrasion values, generally less than 50 mg., and usually below 30 mg.
It is desirable to be able to ship high solids slurries of calcined clay pigments in tank cars. A high degree of fluidity is required as it is with conventional hydrated kaolin pigments. In many instances, slurry shipments must be sufficiently fluid to flow out of tank cars under the influence of gravity alone. Such phenomena as thickening, gel formation, and sedimentation are undesirable because they impair or prevent gravity flow.
When the kaolin is not calcined and has a limited content of particles larger than 2 micrometers (equivalent spherical diameter), it relatively simple to produce a stable high solids (70 percent) suspension of the clay. A powerful anionic deflocculant such as a polyacrylate salt or tetrasodium pyrophosphate (TSPP) is added to a filter cake of acid negatively charged particles of clay, the cake being at about 60 percent solids. Additional dry clay is incorporated with agitation until the suspension has the desired high solids content. The TSPP is usually employed in an amount within the range of 0.3 percent to 0.5 percent based on the dry clay weight. This corresponds to the use of 6 to 10 lbs. TSPP/ton of clay. Typically, the pH of such slurries is in the range of 6.5-8.5. Such suspensions are stable in the sense that there is minimal settling of particles to form a dense sediment and minimal formation of a clear or cloudy supernatant liquid layer when the suspension is allowed to stand. This is attributable to the fact that suspensions of the fine kaolin clay are fairly viscous and contain only small amounts of coarse particles. Few particles of clay, if any, have sufficient mass to settle under the influence of gravity.
However, when clay pigments contain significant amounts of coarse particles, especially particles larger than 2 micrometers, and the content of ultrafine particles is low, there is a marked tendency of coarse particles to settle out of deflocculated suspensions of the clay. For example, 70 percent solids deflocculated suspensions of filler grades of hydrated kaolin clay tend to form hard sediments during shipment or storage. These filler clays usually contain at least 20 percent by weight of particles larger than 5 micrometers and at least 35 percent larger than 2 micrometers.
High solids deflocculated suspensions of calcined clay pigments having particle size distributions similar to those of uncalcined filler clays tend to form hard sediments during storage. Furthermore, calcined clay pigments have unusual rheological properties and the problem of producing stable high solids suspensions is even more difficult than when a typical uncalcined clay is involved. Relatively coarse particle size calcined kaolin clay products such as SATINTONE.RTM. clay usually cannot even be prepared into suspensions containing more than 60 percent solids by conventional techniques without producing systems which are highly dilatant. In the case of ultrafine low abrasion grades, such as ANSILEX.RTM. pigment and SPECTRAFIL.RTM. pigment, fluid suspensions containing more than about 50 percent solids cannot be prepared without impairing the opacifying capacity of the material by subjecting the pigment to excessive mechanical action in dry or wet state. Dilatant systems obtained by slurrying ultrafine particle size grades of calcined kaolin clay resemble quicksand. When a stirring rod is dropped into a fluid concentrated slurry of calcined clay, it may be impossible to extricate the stirring rod unless the stirring rod is removed very slowly. The shearing force applied to the suspension results in the conversion of the originally fluid system into a mass which becomes increasingly viscous as the rate of shear increases. Processing equipment such as mixers and pumps would be damaged by such highly dilatant suspensions or the equipment would stop operating.
A conventional method of maintaining various particulate solids in suspension in fluid media is to thicken the suspending media with suitable colloidal additives. This principle has been advocated to prevent sedimentation in high solids suspensions of filler grades of uncalcined clay. In accordance with the teachings of U.S. Pat. No. 3,130,063 to Millman et. al., an organic polymeric thickening agent, preferably CMC, is added to a previously deflocculated suspension of coarse filler clay in amount sufficient to thicken (and thereby stabilize) the suspension. Anionic dispersants (deflocculating agents) are used. However, organic polymers such as CMC are subject to bacterial degradation. Consequently, clay slurries stabilized with such polymers may arrive at their destination in the form of gray or black masses having a putrid odor. Obviously, it is desirable to avoid stabilizing a deflocculated clay suspension with such thickening agents for the reason alone that preservatives are costly.
It has been suggested (U.S. Pat. No. 3,014,836 to Proctor) to reduce the viscosity of calcined clay pigments by milling the calcined clay under wet or dry conditions. The preferred procedure, as set forth in the patent, is to deflocculate a 55 percent to 60 percent solids suspension of the calcined clay with a conventional amount of a dispersant (0.3 percent TSPP) and ball mill the suspension for 12 to 24 hours. The slip of ballmilled clay is then flocculated by adding acid or alum. The flocculated calcined clay is subsequently dried and then is mixed with water and dispersing agent to produce a 70 percent solids suspension. Proctor did not attempt to produce directly the desired 70 percent solids suspensions of calcined clay and he was not concerned with the sedimentation properties of his suspensions. Furthermore, Proctor did not address the problem resulting from the fact that milling would impair the opacifying properties of the clay.
Similarly, U.S. Pat. No. 3,754,712 to Cecil is concerned with a method for preparing fluid high solids suspensions of calcined clay which are stable without the necessity of adding colloidal thickening agents. Cecil's process involves pebble milling a slurry of anionically dispersed calcined clay and gradually adding more clay to increase solids while the slurry is being milled. Cecil et. al. did not consider the fact that the milling impaired opacification. See also U.S. Pat. Nos. 4,118,245, (Hamil, et. al.) and 4,118,246 (Horzepa et. al.) Among the known dispersants disclosed in U.S. Pat. No. 4,118,246 are condensed phosphate, amino hydroxy compounds such as 2-amino, 2-methyl, 1-propanol (AMP), sodium citrate and sodium naphthalene formaldehyde condensates, alone or in combination.
Marchetti et. al., U.S. Pat. No. 4,118,247 addresses the problem unique to the preparation of slurries of acidic, acid-treated montmorillonite clay pigments. A combination of condensed phosphate and AMP or other amino alcohol is used as the dispersant. In a preferred embodiment, the slurries also contain calcined kaolin clay pigments in major or minor amounts.
In accordance with U.S. Pat. No. 4,107,325 to Eggers, aqueous slurries containing 50 percent or more of calcined clay are prepared by employing a mixture of the calcined clay with a significant amount of uncalcined kaolin clay. Practice of the invention necessitates the use of large amounts of additives including dispersants (and suspending agents). This technique necessitates dilution of the calcined clay with substantial amounts of hydrated clay and thus limits the utility of the products for some end use applications. Furthermore, the high solids slurries were undesirably dilatant.
U.S. Pat. No. 3,804,656 to Kaliski teaches the use of combinations of normally-used anionic dispersants along with nonionic surfactants and cationic surfactants to provide stable pigment slurries. The slurries are only usable at high pH, at least 8 or higher, and preferably 8.5 to 11. In some cases pH values as high as 13 are stated to be desirable.
Commonly assigned U.S. Pat. No. 5,006,574, Sennett teaches a method for preparing a fluid stable aqueous slurry of particles of a calcined clay pigment suitable for use in coating or filling paper or as an extender pigment in paint. The processing includes the steps of providing dispersant-free particles of calcined kaolin clay and adding the particles to a solution of a water-soluble polymeric cationic dispersant in amount sufficient to impart a positive zeta potential to the particles of pigment and to result in a pH in the range of about 3 to 4. The dispersant is selected from the group consisting of alkyl diallyl quaternary ammonium salts, quaternary ammonium cationic polyelectrolytes obtained by copolymerizing aliphatic secondary amines with epichlorohydrin and poly (quaternary ammonium) polyester salts.